keymap: Make replace_termcodes and friends accept length and cpo_flags

Reasons:
- One does not have to do `s[len] = NUL` to work with these functions if they do
  not need to replace the whole string: thus `s` may be const.
- One does not have to save/restore p_cpo to work with them.

src/nvim/api/vim.c

@@ -116,8 +116,14 @@ String vim_replace_termcodes(String str, Boolean from_part, Boolean do_lt,
   }
 
   char *ptr = NULL;
-  replace_termcodes((char_u *)str.data, (char_u **)&ptr,
-                                            from_part, do_lt, special);
+  // Set 'cpoptions' the way we want it.
+  //    FLAG_CPO_BSLASH  set - backslashes are *not* treated specially
+  //    FLAG_CPO_KEYCODE set - keycodes are *not* reverse-engineered
+  //    FLAG_CPO_SPECI unset - <Key> sequences *are* interpreted
+  //  The third from end parameter of replace_termcodes() is true so that the
+  //  <lt> sequence is recognised - needed for a real backslash.
+  replace_termcodes((char_u *)str.data, str.size, (char_u **)&ptr,
+                    from_part, do_lt, special, CPO_TO_CPO_FLAGS);
   return cstr_as_string(ptr);
 }